1. Field of the Invention
The invention relates to a method of forming a magnetic resonance image of an object which is arranged in a steady magnetic field, which method involves repeated execution of an image pulse sequence and includes the following steps:
signals are measured along a first number of lines in the k-space by application of gradients, the method also including the following steps:
2. Description of the Related Art
MR image. The invention also relates to a device for carrying out such a method.
In the context of the present patent application a gradient is to be understood to mean a magnetic field gradient of the steady magnetic field.
The method of the kind set forth is known from the article "The Use of Intelligent Re-acquisition to Reduce Scan Time in MRI degraded by Motion", Q. Nguyen et al., proceedings SMRM 1998, page 134. The known method can be used for the in vivo imaging of diffusion phenomena in tissue of a human or animal body to be examined, for example a part of the brain. Diffusion-weighted magnetic resonance (MR) images may be of assistance for making diagnoses of diseases, for example brain infarcts, and for characterizing brain tumors. In order to enable the imaging of diffusion phenomena, the known method also includes a magnetization preparation pulse sequence which includes a gradient pair and a refocusing RF pulse in order to realize an amplitude modulation in the MR signals received, said amplitude modulation being dependent on diffusion of material in the part to be selected in the brain of the body to be examined. The imaging pulse sequence succeeding the magnetization preparation pulse sequence yields navigator MR signals and MR signals wherefrom the MR image of the part of the tissue is reconstructed. MR signals are measured along the number of lines in the k-space as a result of the generation of RF pulses and the application of magnetic field gradients. Repeating the imaging pulse sequence ensures that the imaging MR signals are measured along a number of lines in the k-space which is large enough to ensure that diagnostically useful MR images become available after reconstruction. The applied gradients serve for slice selection, phase encoding and frequency encoding of the MR signals. After all imaging MR signals have been measured, the MR image of the part of the brain to be examined is reconstructed from the measured imaging MR signals while utilizing, for example a two-dimensional Fourier transformation. Subsequently, an imaging quality of the reconstructed MR image is determined. According to the known method an imaging quality of this kind is determined from a ratio of a mean value of intensities of pixels within a boundary of an imaged part of the object to a mean value of intensities of pixels within a boundary of a ghost image of the imaged part of the object. In the context of the present patent application a ghost image is to be understood to mean an image of the imaged part which has been shifted relative to the image and has a lower intensity. If the imaging quality is too low, according to the known method the MR image is corrected by replacing at least one of the measured imaging MR signals by an MR signal to be newly measured for said imaging MR signal, a magnitude of the associated navigator MR signal being smaller than or equal to the magnitudes of the navigator MR signals associated with the other measured imaging MR signals. The imaging MR signal to be measured anew is measured along the same trajectory in the k-space as that along which the imaging MR signal to be replaced has been measured. Subsequently, a new MR image is reconstructed again from the imaging MR signals and the imaging quality of this new MR image is determined again. Subsequently, said steps are repeated until the imaging quality of the MR image is satisfactory.
It is a drawback of the known method that the imaging quality of the MR image is determined after the final reconstruction. It is a further drawback that the imaging quality cannot be unambiguously defined, because the boundary of the imaged part of the object and the boundary of its ghost image can be determined from the reconstructed MR image only afterwards.
Citation of a reference herein, or throughout this specification, is not to construed as an admission that such reference is prior art to the Applicant's invention of the invention subsequently claimed.